The present invention relates to a bias circuit and a method of fabricating a bipolar integrated circuit in which elements of the bias circuit are integrated.
Recently, a field effect transistor formed from GaAs with small power consumption (MESFET) is widely used as a transistor of a transmitting power amplifier used in mobile communication equipment such as a portable telephone. A negative power source is generally used for bias for a gate electrode of a MESFET. Accordingly, in using a MESFET in a transmitting power amplifier, two power sources, namely, a positive power source and a negative power source, are required. This is a disadvantage to downsizing of the amplifier, and hence, a transistor operated by a positive power source alone is earnestly desired.
Furthermore, in recent communication systems such as CDMA (code division multi-channel access), an output current of a transmitting power amplifier is required to have small distortion (namely, to be linear). As a transistor meeting these requirements, a heterojunction bipolar transistor (HBT) including the emitter formed from a semiconductor having a larger band gap than a semiconductor forming the base is practically used.
In a conventional power amplifier using HBTs, a bias circuit is generally constructed on the same chip for supplying a current necessary for the base of an HBT used as a power transistor. An HBT has, however, a characteristic that the on state voltage decreases as the temperature increases as is shown in FIG. 10 (which characteristic is hereinafter referred to as the temperature characteristic of an HBT). Therefore, when a given voltage is applied between the base and the emitter, a collector current (hereinafter referred to as the idle current) of the HBT is largely increased as the temperature increases. Accordingly, the bias circuit is required to reduce change with temperature of the idle current of the HBT serving as the power transistor.
A bias circuit for overcoming the problem will now be described with reference to FIG. 11, which shows a bias circuit 100 used in a conventional power amplifier.
The base terminal of a bipolar transistor Tr101 serving as a power transistor is connected through a resistor R103 of 4 Ω to a bipolar transistor Tr102 so as to compose an emitter-follower circuit. Also, the base terminal of the transistor Tr102 is grounded through transistors Tr103 and Tr104 in each of which the base and the collector are short-circuited. The transistors Tr103 and Tr104 are PN diodes having the same on state voltage as the transistors Tr101 and Tr102. When the temperature is increased in this circuit, the idle current C of the transistor Tr101, that is, the HBT, is increased owing to the temperature characteristic. On the other hand, a current flowing through the transistors Tr103 and Tr104 is also increased owing to the same temperature characteristic. Accordingly, a current flowing through a resistor R101 connected to the transistors Tr103 and Tr104 in series is increased. Since the resistance of the resistor R101 is constant (530 Ω), a voltage applied to the resistor R101 is increased as the current increases. In other words, a potential at a point P5 of FIG. 11 is lowered. Accordingly, the base potential of the transistor Tr102 connected to the resistor R101 is lowered. As a result, the emitter current of the transistor Tr102 is decreased, so as to lower the base potential of the power transistor Tr101. In this manner, the idle current C of the power transistor Tr101 can be suppressed from increasing.
The bias circuit 100 of FIG. 11 thus suppresses the idle current C of the power transistor Tr101 from increasing in accordance with the temperature increase.
In the conventional bias circuit 100, however, the suppression of the change of the idle current is disadvantageously insufficient.